No doubt the intensity of the
attraction becomes weaker with every increase in the altitude, but
that action would still exist to some extent, however lofty might be
the elevation which had been attained.
It then occurred to Newton, that though the moon is at a distance of
two hundred and forty thousand miles from the earth, yet the
attractive power of the earth must extend to the moon. He was
particularly led to think of the moon in this connection, not only
because the moon is so much closer to the earth than are any other
celestial bodies, but also because the moon is an appendage to the
earth, always revolving around it. The moon is certainly attracted
to the earth, and yet the moon does not fall down; how is this to be
accounted for? The explanation was to be found in the character of
the moon's present motion. If the moon were left for a moment at
rest, there can be no doubt that the attraction of the earth would
begin to draw the lunar globe in towards our globe. In the course of
a few days our satellite would come down on the earth with a most
fearful crash. This catastrophe is averted by the circumstance that
the moon has a movement of revolution around the earth. Newton was
able to calculate from the known laws of mechanics, which he had
himself been mainly instrumental in discovering, what the attractive
power of the earth must be, so that the moon shall move precisely as
we find it to move.
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